The approach to equilibrium in a buoyant-density gradient.

The approach to equilibrium in a buoyant-density gradient is a transport process in a centrifugal field and may be used to measure a sedimentation coefficient. This process is interesting for several reasons. Densitygradient sedimentation equilibrium can be used for the determination of the molecular weight of high-molecular weight DNA samples homogeneous in buoyant density. Furthermore, buoyant-density centrifugation, in one form or another, has been one of the most important techniques in DNA experiments. With an understanding of the approach to equilibrium it would be possible to obtain a molecular weight, buoyant density, and sedimentation coefficient in the same experiment. A two-component theory for the approach to equilibrium was developed some time ago with considerable emphasis on the effects of both molecular weight and density heterogeneity. l v 2 I n these early efforts considerable attention was also focussed on alternate methods of data analysis. However, an adequate description of this problem also requires consideration of the multicomponent nature of the system. Typically, the system might consist of three components: macromolecule, salt, and water. Hearst3 reconsidered this problem along lines similar t o the original contributions, but with a proper description of the multicomponent system. As could be an t i~ ipa ted ,~ the thermodynamic data required to evaluate the approach to equilibrium are the same as those required to evaluate the equilibrium distribution. Accurate thermodynamic data for DNA in buoyant cesium salts are now available5 and may be used to test this multicomponent theory.